The United States has an almost unimaginable volume of hydrocarbons locked up in huge reserves of oil shale. However, the current technology for recovering oil from oil shale has been a substantially expensive process that is only economically viable when crude oil prices are high. Shale does not have the energy density of coal with more than about 85% being rock.
While mining the shale is discouraged by the cost, another technique for liberating the hydrocarbons has been to create an underground retort or reaction vessel and heating shale to a temperature that cracks the hydrocarbon and releases a shale oil. The process of creating the retort begins by defining the area that one wants to work with. Keep in mind that the shale deposits can extend horizontally for hundreds of miles. So, for instance, an in-situ retort may be defined as perhaps 200 feet by 400 feet and by the thickness of the oil shale deposit which can vary between 200 feet and up to 2,000 feet thick. Within the retort area, about twenty percent of the shale deposit is removed or mined out and the remaining portion has boreholes drilled in a pattern for an explosive to convert the solid shale formation into rubble. This rubbilizing process opens up the shale so that it can be heated by the circulation of hot gases and a drain is installed at the base of the retort to collect the shale oil. The heat of the process also creates gases including hydrocarbons which are collected through a separate recovery device.
Into the top of the retort, air is injected and a combustion process is ignited. The retort includes an overburden so the top of the retort is not open to the atmosphere. The air injection provides enough oxygen for combustion, but does not burn all of the hydrocarbons. Indeed, the remaining gases in the retort are oxygen depleted so the hydrocarbons are thermally cracked and vaporized. Some of this product condenses into hydrocarbon liquids when cooled in lower regions of the retort. It then drains by gravity to recovery equipment at the bottom of the retort. This process may burn for many months, but the temperature control is quite poor and the retort can easily reach very high temperatures up to and including 1,700° F. This process was developed before concerns were substantially raised about the liberation of carbon dioxide and other greenhouse gases. The rock which holds the kerogen is substantially comprised of metal carbonates such as calcium carbonate, aluminum carbonate. If these rock materials are heated in excess of 1,100 F, they decompose into metal oxides and carbon dioxide. Many tons of carbon dioxide could easily be released by processing of one retort of the size described and the carbon would not be from the small amount of hydrocarbon present.
Clearly, technology for economically recovering oil shale would be very helpful to the United States in terms of American energy independence, but recovery of the oil shale must be accomplished in an environmentally safe and sensitive manner for the technology to be practically applied.